1. Field of the Invention
Embodiments of the disclosure relate to a liquid crystal display and a local dimming control method thereof capable of improving image quality.
2. Discussion of the Related Art
The range of applications for liquid crystal displays has gradually widened because of their excellent characteristics such as light weight, thin profile, and low power consumption. Liquid crystal displays have been used in personal computers such as notebook PCs, office automation equipments, audio/video equipments, interior/outdoor advertising display devices, and the like. The liquid crystal displays display an image using a thin film transistor (TFT) as a switching element. A backlit liquid crystal display occupying most of the liquid crystal displays controls an electric field applied to a liquid crystal layer and modulates light coming from a backlight unit, thereby displaying an image.
The image quality of the liquid crystal display depends on its contrast characteristic. Improvements to the contrast characteristic are limited using only a method for controlling the data voltage applied to the liquid crystal layer and modulating a light transmittance of the liquid crystal layer. As a solution, a backlight dimming method has been proposed so as to improve the contrast characteristic. The backlight dimming method adjusts the luminance of a backlight unit depending on an image displayed on the liquid crystal display. The backlight dimming method includes a global dimming method for adjusting a luminance of the entire display surface of the liquid crystal display and a local dimming method for locally controlling a luminance of the display surface of the liquid crystal display. The global dimming method can improve a dynamic contrast ratio measured between two adjacent frames. The local dimming method can locally control the luminance of the display surface of the liquid crystal display within one frame period, thereby improving a static contrast ratio which is difficult to improve using the global dimming method.
In the local dimming method, the backlight unit is divided into a plurality of blocks. The local dimming method adjusts a dimming value of each of the plurality of blocks, thereby increasing a luminance of the backlight unit belonging to the block displaying a bright image and reducing a luminance of the backlight unit belonging to the block displaying a dark image. In other words, a plurality of light sources of the backlight unit are partially turned on in local dimming. Thus, the luminance of the backlight unit when the local dimming method is applied is less than the luminance of the backlight unit when the local dimming method is not applied (i.e., when all of the light sources of the backlight unit are turned on). A modulation of pixel data may compensate for a deficiency of the luminance of the backlight unit resulting from the local dimming method. The modulation of the pixel data is performed based on a pixel gain value according to the result of an analysis of an amount of light of the backlight unit of each block.
The pixel gain value is determined based on data required to allow a luminance obtained from a total amount of light (i.e., an amount of light in dimming) reaching a corresponding pixel in the local dimming using a dimming value of a block including the corresponding pixel to be equal to a luminance obtained from a total amount of light (i.e., an amount of light in non-dimming) reaching the corresponding pixel in non-local dimming. The pixel gain value is calculated by a ratio of the amount of light in the non-dimming to the amount of light in the dimming. When the amount of light in the dimming is less than the amount of light in the non-dimming, the pixel gain value in the corresponding pixel increases. As the pixel gain value increases, an upward modulation width of the data increases. As a result, grayscale saturation occurs in which high gray levels appear at the same brightness. As the grayscale saturation becomes serious, the image quality of the liquid crystal display is degraded. Accordingly, a method for controlling the pixel gain value based on a grayscale saturation level is required.